Uranyl-Selective Electrode Based on a New Bifunctional Derivative

Mar 7, 2000 - (15) Warshawsky A.; Kahana, N.; Arad-Yellin, R. Hydrometallurgy 1989, ... (16) Warshawsky A.; Arad-Yellin, R. YEDA Co., Israel, Israeli ...
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Anal. Chem. 2000, 72, 1604-1610

Uranyl-Selective Electrode Based on a New Bifunctional Derivative Combining the Synergistic Properties of Phosphine Oxide and Ester of Phosphoric Acid Antonio Florido,*,† Ignasi Casas,† Josep Garcı´a-Raurich,‡ Rina Arad-Yellin,§ and Abraham Warshawsky§

Departament d’Enginyeria Quı´mica, ETSEIB, Universitat Polite` cnica de Catalunya, 08028 Barcelona, Spain, Departament d’Enginyeria Quı´mica, EUETIT, Universitat Polite` cnica de Catalunya, 08222 Terrassa, Spain, and Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel

Ion-selective electrodes based on the bifunctional chelating agent O-methyldihexylphosphine oxide O′-hexyl-2ethylphosphoric acid (HL) incorporated into a poly(vinyl chloride) membrane were developed. This new derivative is proposed as a single molecular unit combining the overall properties of the synergistic single components, di-2-ethylhexylphosphoric acid and trioctylphosphine oxide. Two different ionophores, HL and its uranyl complex (UO2L2), were studied. The response of the electrodes to uranyl ion was Nernstian for UO2L2 and super-Nernstian for HL ionophores, with detection limits of 3.0 × 10-6 and 2.0 × 10-5 M, respectively. Results indicate a more effective interaction with the analyte in the case of having a unique molecule incorporating the two functional groups immobilized into a polymeric membrane, rather than the separated two synergistic ligands. Flow-through tubular electrodes based on both ionophores were also used as potentiometric detectors in flow injection techniques. Uranium dioxide is used in the preparation of fuel pellets for nuclear power reactors. Several steps are necessary in this process: leaching from ores, purification by ion-exchange and solvent extraction, precipitation, reduction, etc. The monitoring of uranium concentration in all these process streams is essential (i.e., efficiency of the extraction or the stripping, etc).1 On the other hand, considerable interest has developed in on-site environmental monitoring of uranium. As an example, uranium monitoring is an important parameter in the characterization of natural analogue systems used for the safety assessment of future geological disposal of radioactive wastes.2 Thus, the determination * Corresponding author: (tel) +34-934016554; (fax) +34-934015814; (e-mail) [email protected]). † Departament d’Enginyeria Quı´mica, ETSEIB, Universitat Polite ` cnica de Catalunya. ‡ Departament d’Enginyeria Quı´mica, EUETIT, Universitat Polite ` cnica de Catalunya. § The Weizmann Institute of Science. (1) Narasimha Murty, B.; Jagannath, Y. V. S.; Yadav, R. B.; Ramamurty, C. K.; Syamsundar, S. Talanta 1997, 44, 283-95. (2) Miller, W.; Alexander, R.; Chapman, N.; McKinley, I.; Smellie, J. Natural analogue studies in the geological disposal of radioacticve wastes; Elsevier Science B.V.: Amsterdam, 1994.

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of uranium in these process streams on a routine basis or for the immediate detection of sudden uranium contamination is necessary. Several analytical methods, such as spectrophotometric and fluorometric analyses, were used for uranium determination. Taking into account the above-mentioned situations, the following characteristics are highly desirable: covering a wide range of concentrations, prior separation from the impurities not necessary, sensitivity, quickness, simplicity, adequate accuracy and precision, and cost-effectiveness. For these reasons, sensors are convenient, and the fact that several new uranyl optical and electrochemical sensors have been reported recently 3-7 shows that there is still a need for improved uranyl sensors. Specifically, uranyl-selective electrodes are highly convenient for routine process and field applications, their most important advantage over other analytical techniques being that they can measure uranium concentration over several orders of magnitude. Moreover, the combination of these sensors with flow injection techniques (especially advantageous for sample pretreatment, preconcentration or dilution, and interference separation) makes them suitable for real-time monitoring of cleanup studies or in process streams. On the other hand, continuous automated analytical techniques are preferred over batch methods when dealing with hazardous materials or when a large number of samples have to be analyzed. For the chemical recognition of uranyl ions, cyclic or acyclic, charged or neutral ionophores have been used, most of them being organophosphorus compounds.5,6,8-14 Most of these organophosphorus compounds, which give a potential change on selec(3) Lerchi, M.; Reitter, E.; Simon, W. Fresenius J. Anal. Chem. 1994, 348, 272-76. (4) Savvin, S. B.; Dzherayan, T. G.; Petrova, T. V.; Mikhailova, A. V. J. Anal. Chem. 1997, 52, 136-40. (5) Saleh, M. B. Indian J. Chem. 1992, 31, 12-6. (6) Yoshida, Z.; Aoyagi, H.; Meguro, Y.; Kitatsuji, Y.; Kihara, S. J. Alloys Comput. 1994, 213/214, 324-7. (7) Dumkiewicz, R. Talanta. 1994, 41, 2183-8. (8) Goldberg, I.; Meyerstein, D. Anal. Chem. 1980, 52, 2105-8. (9) Lubert, K. H.; Schnurrbusch, M.; Thomas, A. Anal. Chim. Acta. 1982, 144, 123-36. (10) Luo, C. S.; Chang, F. C.; Yeh, Y. C. Anal. Chem. 1982, 54, 2333-6. (11) Serebrennikova, N. V.; Kukushkina, I. I.; Plotnikova, N. V. J. Anal. Chem. USSR 1982, 37, 485-9. (12) Moody, G. J.; Slater, J. M.; Thomas, J. D. R. Analyst 1988, 113, 699-703. (13) Nassory, N. S. Talanta 1989, 36, 672-4. 10.1021/ac990806l CCC: $19.00

© 2000 American Chemical Society Published on Web 03/07/2000

tive complexation with uranium, were also applied extensively in extraction studies. In the last decade, a great effort was made toward the synthesis of new classes of extractants and chelating exchangers capable of improving the efficiency and selectivity of a number of separation processes for a wide range of chemical species. These compounds have been used for the preparation of new ionophores, and moreover, the knowledge obtained from complexation data can be used in the design of new sensing devices. Thus, uraniumselective electrodes based on the commercial extractants of uranium di-2-ethylhexylphosphoric acid (D-2EHPA) and trioctylphosphine oxide (TOPO) or the combination of both have been reported.10,11 In such cases, various problems in the electrode response or only fair response characteristics were observed, and moreover, membranes prepared with the combination of these two ion exchangers resulted in erratic or poor results. On the other hand, the synergistic activity of the commercial reagents D-2EHPA and TOPO for the efficient extraction of uranium from phosphoric acid solutions is well known, and this synergistic phenomenon has been used in the improvement of these sensor systems for the determination of uranium.10,11 The mechanism of the synergistic extraction involves the formation of a trimolecular complex TOPO-UO2-D-2EHPA, where TOPO is responsible for pulling UO22+ from the aqueous phase, and D-2EHPA improves the solubility of the complex in the organic phase.15 A bifunctional compound, O-methyldihexylphosphine oxide O′-hexyl-2-ethylphosphoric acid, that possesses the properties of the two components of the above synergistic mixture in one entity has been prepared.15-17 The linking of phosphine oxide and phosphate groups in the same molecule has resulted in an enhancement of the extraction capabilities (8.7 times stronger), due to the formation of a bimolecular complex (i.e., the extraction mechanism steps are simpler).15 The synergistic effect of the two moieties of the molecule has been demonstrated as well by 31P NMR, where it was observed that both phosphorus ligands participate in the complexation.17 This new reagent shows a considerable advantage in the extraction of U(VI) over synergistic mixtures of extractants in solvent extraction processes. In addition, this organophosphorus derivative have been used in liquid-liquid and solid-liquid distribution studies of uranium and some divalent metals, and it has been also covalently immobilized on a polymeric matrix.15,16,18,19 In the present work, the bifunctional chelating agent, Omethyldihexylphosphine oxide O′-hexyl-2-ethylphosphoric acid (HL) (Figure 1), incorporating a phosphine oxide and phosphoric acid diester functionalities, and its uranyl complex (UO2L2), both synthesized in our laboratories,15-17 are proposed as ionophores for the determination of uranyl ion. Flow-through tubular electrodes based on both ionophores are also studied as potentio(14) Petrukhin, O. M.; Avdeeva, E. N.; Zhukov, A. F.; Polosuchina, I. B.; Krylova, S. A.; Rogatinskaya, S. L.; Bodrin, G. V.; Nesterova, N. P.; Polikarpov, Y. M.; Kabachnik, M. I. Analyst 1991, 116, 715-9. (15) Warshawsky A.; Kahana, N.; Arad-Yellin, R. Hydrometallurgy 1989, 23, 91104. (16) Warshawsky A.; Arad-Yellin, R. YEDA Co., Israel, Israeli Patent Application, 79-999 and 80-000, 1986. (17) Arad-Yellin, R.; Zangen, M.; Gottlieb, H.; Warshawsky, A. J. Chem. Soc., Dalton Trans. 1990, 1990, 1-8. (18) Cortina, J. L.; Miralles, N.; Aguilar, M.; Warshawsky, A. React. Funct. Polym. 1995, 27, 61-73. (19) Arad-Yellin, R.; Warshawsky, A. React. Polym. 1989, 11, 21-4.

Figure 1. Chemical structure of the ligand O-methyldihexylphosphine oxide O'-hexyl-2-ethylphosphoric acid (HL). Table 1. Composition of Membrane Mixtures Used To Prepare the ISEs membrane composition

ionophore (mg) plasticizer (mg) PVC (mg) KTCPB (mg)

A

B

C

D

E

F

2 66 32 0

2 66 31 1

4 66 30 0

4 66 29 1

4 66 28.5 1.5

4 66 28 2

metric detectors by flow injection techniques. Moreover and as a general procedure, what it is proposed in this work is that, when the synergistic effect of two compounds is well known, it is possible to prepare a new derivative with the active part of both and use it as an ionophore in the development of a new sensor. EXPERIMENTAL SECTION Reagents and Apparatus. All chemicals were of analytical reagent grade. Bis(2-ethylhexyl) sebacate (DOS), 2-nitrophenyl octyl ether (NPOE), bis(2-ethylhexyl) phthalate (DOP), dioctyl phenylphosphonate (DOPP), tris(2-ethylhexyl) phosphate (TOP), dibutyl phthalate (DBP), and chromatographic grade poly(vinyl chloride) (PVC) were purchased from Fluka (Buchs, Switzerland). Tetrahydrofuran (THF), potassium tetrakis(4-chlorophenyl)borate (KTCPB), tris(hydroxymethyl)aminomethane (Tris), and inorganic salts were from Merck (Darmstadt, Germany). Either the silver-loaded epoxy resin Epotek 410, from Epoxy Technology (Billerica, MA), or a mixture of graphite powder, from Merck, and nonconductive epoxy resin (Araldite M and HR hardener), from Ciba-Geigy (Barcelona, Spain), were used as conductive membrane supports. All standard solutions and buffers were prepared with deionized water using a Milli-Q unit (Millipore, Bedford, MA). Potentiometric responses were measured at room temperature with two different digital pH/mV meters (Crison, model Micro pH 2002, Alella, Spain; MBT Environmental, model 6122, Cerdanyola, Spain) and registered on a Linseis (model L6512B, Selb, Germany) strip-chart recorder. In flow injection studies, solutions were pumped by a Gilson peristaltic pump (model Minipuls-3, Villiers-le-Bel, France), and samples were injected using an Omnifit four-way manual valve. Teflon tubing (0.5 mm id) was used for the construction of the flow system. Synthesis of the Ionophores. The ligand HL was prepared according to a published procedure.15,16 The uranium(VI) complex of HL was prepared as specified previously17 under conditions favorable to the formation of the 1:2 complex. The complex UO2L2 was obtained as a light yellow solid. Membranes and Electrodes. Membranes were prepared by mixing the ionophore (HL or UO2L2), the plasticizer, the KTCPB, and the polymer (PVC) at the composition shown in Table 1. The mixture was dissolved in 1 mL of THF and then two different electrode configurations were prepared. In electrodes with internal Analytical Chemistry, Vol. 72, No. 7, April 1, 2000

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reference solution, the sensor mixture was cast in a 16-mm-i.d. glass ring.20 The solvent was allowed to evaporate overnight, leaving an approximately 0.3-mm-thick membrane from which small-diameter disks were cut and mounted in a Philips IS-581 or Fluka, ref 45137, electrode body. The internal filling solution was 0.010 M KCl. In flow-through tubular electrodes, the mixture was added dropwise inside the 1.75-mm-i.d. channel drilled in the membrane support. The solvent was allowed to evaporate, and the procedure was repeated until membranes of appropriate thickness were obtained. Two different composites were used as conductive membrane support: a silver-loaded epoxy resin, and a conductive material prepared from a nonconductive epoxy resin and graphite powder mixed in the weight ratio of 1:0.9. Membranes were conditioned in either 0.001 M HNO3 or 1.0 × 10-3 M UO22+ in 0.001 M HNO3, or they were kept dry. Of the different possibilities, keeping the membranes dry was found to be the most suitable. An Orion double-junction electrode (model 90-02-00) was used as reference electrode. The Orion solution (ref 90-00-02) was used as internal filling solution, and the outer compartment was filled with the same ionic medium as employed in the test or in the carrier solutions (see below). Procedure. Different volumes of standard solutions of a series of several electrolytes (the principal ion or potential interferences) were added to 10 mL of either 0.001 M HNO3 or 0.100 M TrisHNO3, at pH 3. Differences in potential were measured with the pH/mV meter and registered with the strip-chart recorder. Calibration parameters were obtained by plotting the measured potential vs the logarithm of the concentration of the analyte in the sample solution. All experiments were performed at room temperature. Flow Injection Studies. The manifold used in the flow injection experiments has been described elsewhere.21 Flow parameters were optimized in order to obtain the best signal sensitivity and sampling rate under low dispersion conditions. The optimized parameters were as follows: injection volume 100-150 µL (see below); carrier flow rate 3.2-3.9 mL/min; sample flow rate 1.7-2.0 mL/min; and length of tubing between injection valve and detector 29 cm. Different solutions were used as a carrier: (a) 1.0 × 10-3 M HNO3; (b) 1.0 × 10-3 M HCl; (c) 1.0 × 10-3 M HClO4; (d) 0.100 M (K+, H+) NO3-, pH 3; (e) 0.100 M (Na+, H+) Cl-, pH 3; (f) 0.100 M (Na+, H+) ClO4-, pH 3. To stabilize the baseline, all carrier solutions contained 5.0 × 10-6 M UO22+. Uranyl standard solutions in the range 1.0 × 10-5-1.0 × 10-1 M were prepared in each of the media described above (a-f). The sample was injected into the carrier, and changes in concentration with respect to analyte background were measured with the flow-through tubular electrode as changes in potential relative to the baseline. Calibration graphs were plotted as peak heights (in mV) vs the logarithm of the activity of analyte in the sample solution. Due to the large number of aqueous species that uranium may form with the different ions in solution, a careful control of the experimental conditions was needed. In all flow experiments, a detailed calculation of the aqueous species activities was performed. In this sense, the activity of uranyl was calculated (20) Moody, G. J.; Thomas, J. D. R. In Ion Selective Electrodes in Analytical Chemistry; Freiser, H., Ed.; Plenum Press: New York, 1981; p 143. (21) Daunert, S.; Florido, A.; Bricker, J.; Dunaway, W.; Bachas, L. G.; Valiente, M. Electroanalysis 1993, 5, 839-43.

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Table 2. Effect of the Plasticizer on the Membranes plasticizer DOPP DOP

slope (mV/decade)

PDL (M) 10-6

+14 -50

6× 3 × 10-5

+5 -10 +20

1 × 10-5 3 × 10-4 1 × 10-6

NPOE TOP DBP DOS

comments membranes broke easily negative slope and slow response no response; membranes broke easily low slope; slow response negative slope best slope and PDL; fast response

in each case by using the equilibrium geochemical calculation code HARPHRQ,22 together with the recently published Nuclear Energy Agency (NEA) uranium thermodynamic database.23 Total concentrations of all the solution components were used as input. The activity of all species was determined and given in the output of the program. RESULTS AND DISCUSSION Results are presented related to the two different ionophores studied (UO2L2 and HL) and the two different electrode configurations (conventional and flow-through tubular electrodes). UO2L2-Based Electrodes. To find the best plasticizer, different membranes of composition A (see Table 1) and UO2L2 as ionophore were prepared. The obtained results are shown in Table 2. The best calibration parameters and mechanical characteristics of the membranes were observed in the case of DOS; hence, this plasticizer was used in further studies. The influence of pH was also studied. The results indicated that the optimum pH range was 2.5-4. Hydroxo complexes can be formed or precipitate at higher pH values, and membranes also suffer a strong interference from hydrogen ion at pH 80%). Therefore, 0.001 M HNO3 solution was used as a test solution in batch studies. In such cases, the UO22+/Utot ratio was constant in all experiments. For this reason, results are presented directly as a function of the total aqueous uranium concentration. Some deviation from linearity was observed at higher uranium concentrations. The use of an anion excluder has been suggested previously for other PVC matrix membrane uranyl-selective electrodes.5,10,14,24 In Figure 2, the effect of the KTCPB on the electrode response is presented for the comparison between (22) Brown, P. L.; Haworth, A.; Sharland, S. M.; Tweed, C. J. HARPHRQ: A geochemical speciation program based on PHREEQE. Theoretical Studies Department, Radwaste Disposal Division B424.2. Hornwall Laboratories: Didcot Oxon, OX110RA, 1991. (23) Wanner, H., Forest, I., Eds Chemical Thermodynamics of Uranium; NorthHolland Elsevier Science: Amsterdam, 1992; Vol. 1; pp 51-60. (24) Johnson, S.; Moody, G. J.; Thomas, J. D. R.; Kohnke; F. H.; Stoddart, J. F. Analyst 1989, 114, 1025-8. (25) Palmer, D. A.; Nguyen-Trung, C. J. Solution Chem. 1995, 24, 1281-91.

Figure 2. Effect of the KTCPB on the electrode response: (a) without KTCPB (membrane A); (b) with KTCPB (membrane B) (see Table 1).

membrane compositions A and B. One can observe that the incorporation of KTCPB improves the permselectivity of the cations, resulting in a wider operational range. The calibration parameters, response times, and lifetimes of different membranes prepared with the uranium complex (UO2L2) as ionophore, by using the conditions mentioned above, have been determined. Electrodes presented Nernstian slopes (25-31 mV/ decade) in the response to uranyl ion, with detection limits, defined as practical detection limits (PDL),26 (2-4) × 10-6 M. Taking into account that, in industrial solutions, the uranium concentration spans the range from 350 g/L (1.47 M) to as low as 5 mg/L (2.10 × 10-5 M),1 the electrodes presented in this work could be used directly in samples at medium and low concentration levels. However, the high uranium concentration observed in some process solutions would require previous dilution. It is important to point out that the same degree of dilution would be required in other analytical techniques.1 The response time was defined as the time elapsed to attain a response that is within 1 mV of the steady-state signal after addition of the ion to the sample. Response times were found to be between 10 and 30 s, depending on the uranyl concentration in the sample. These response characteristics were maintained during the entire operational lifetime of the electrodes (around 20 days). Similar behavior of the electrode responses was found for the 2 and 4% ionophore ratios. Two blank PVC membranes were also prepared: one contained DOS, and the second, DOS + KTCPB. No uranyl response was observed for these electrodes. In Figure 3, the selectivity pattern obtained with the 4% UO2L2based membranes is presented. This figure indicates that membranes responded preferentially to UO22+ > Fe3+ ≈ F- > H2PO4- . Al3+ > other ions. In these graphs, ∆E is the difference between the steady-state potential and the starting potential (i.e., measured potential before addition of ions). Other studied ions that presented little or no response (Zn2+, Na+, Cl-, Mg2+) are not plotted in the figure. The interference observed in the case of iron(III) is due to the fact that the ligand is also an extractant of this metal.17 However, very little interference was observed in (26) Commission on Analytical Nomenclature. Pure Appl. Chem. 1975, 48, 129-32.

Figure 3. Selectivity pattern of the UO2L2-based electrodes with a 4% ionophore ratio (membrane D). ∆E corresponds to the change in potential from the starting potential (see text). The electrode was exposed to (1) UO22+, (2) Fe3+, (3) F-, (4) H2PO4-, (5) Al3+, (6) Cu2+, (7) K+, (8) Ba2+, (9) Ni2+, (10) Sr2+, (11) Cr3+, (12) Ca2+, (13) NO3-, (14) SO42-, (15) Mn2+, and (16) Co2+.

the case of Cu2+. This agrees with the solvent extraction studies of these metals with the same ligand.15 In any case, it is necessary to point out that uranyl electrodes based on different organophosphates or neutral carriers usually present selectivity to iron(III).12,24 However, either reduction of Fe(III) to Fe(II) or Fe(III) masking by phosphate or ascorbic acid is normally used in industrial processes to avoid its interference.1,4 In general, removal/masking of these interfering ions would be required for samples rich in these ions. On the other hand, interferences caused by fluoride and dihydrogen phosphate can be related to the stable complexes that these anions could form with the U(VI) metal center in the UO2L2-based membrane, as one can observe from the high values of the formation constants of the fluoro- and phosphate-uranium complexes.23 Silica present as suspended particles and iron present as colored complexes, normal impurities in some of the process streams and which gave errors in spectrophotometric determinations,1 would not affect electrode response. Finally, similar selectivity behavior was obtained in electrodes with membrane composition B (2% ionophore content). The response to the UO22+ ions shown by these UO2L2 electrodes can be explained by a mechanism where the concentration of this ion in the sample would affect, through interfacial transport, the level of dissociation of the complex UO2L2 in the membrane.27 POT The potentiometric selectivity coefficients, kUO 2+ , for all the 2 ,Y ions studied, and considering uranyl as the principal ion, are presented in Table 3. These selectivity coefficients were calculated by the matched-potential method.28,29 HL-Based Electrodes. The same membrane components and compositions (B and D) as in the UO2L2-based electrodes were used. In the pH studies, results indicated that a similar operational plateau was obtained as before; consequently the same ionic media and pH were also applied. Membranes containing only the ligand (27) Florido, A.; Bachas, L. G.; Valiente, M.; Villaescusa, I. Analyst 1994, 119, 2421-5. (28) Gadzekpo, V. P. Y.; Christian, G. D. Anal. Chim. Acta 1984, 164, 279-82. (29) Attiyat, A. S.; Kadry, A. M.; Badawy, M. A.; Hanna, H. R.; Ibrahim, Y. A.; Christian, G. D. Electroanalysis 1990, 2, 119-25.

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Table 3. Selectivity Coefficients of the UO2L2-Based Electrodes interference (Y) Fe3+ Mn2+ H2PO4FSr2+ Al3+ Ni2+

POT log kUO ( SD 2+ 2 ,Y

-1.67 ( 0.03 -1.7 ( 0.2 -1.92 ( 0.06 -1.956 ( 0.006 -2.16 ( 0.06 -2.40 ( 0.09 -2.5 ( 0.1

interference (Y) Co2+ K+ Ba2+ Ca2+ NO3Cu2+ Cr3+

POT log kUO ( SD 2+ 2 ,Y

-2.59 ( 0.07 -3.15 ( 0.05 -3.16 ( 0.03 -3.22 ( 0.02 -3.4 ( 0.1 -3.48 ( 0.04 -3.5 ( 0.1

(HL) showed super-Nernstian slopes in the range 74-83 and 70-77 mV/decade, for the 2 and 4% ionophore content, respectively, with detection limits between 1 × 10-5 and 2 × 10-5 M for both compositions. Slower response times (electrodes stabilized between 1 and 4 min, depending on the concentration level) were found in the case of HL as ionophore. The slower response compared with UO2L2based electrodes may be attributed to the response mechanism of these membranes. Thus, the differences in the response times of UO2L2-based electrodes and HL-based membranes is probably due to the different mechanism of molecular interaction between the membrane-immobilized ligands and the UO22+ ion. Even though the membrane potential is influenced by both ionexchange and extraction processes,7 in the case of UO2L2-based membranes, the mechanism of metal sensing is due to an ionexchange reaction between external UO22+ ions and internal UO22+ complexed with the ligand (immobilized in the membrane). In the case of HL-based electrodes, the mechanism involves metal complexation by chelation of external UO22+ ions. It is well known that ion-exchange processes are faster than chelation.30 On the other hand, super-Nernstian slopes obtained in the uranyl response of the HL electrodes cannot be explained by a single mechanism.31 The response of these electrodes is definitively reproducible, but slopes around 70-83 mV/decade could be related to different interactions between the analyte and the membrane (extraction, ion exchange, efficiency of the carrier mechanism, etc.).7,24,31 However, longer lifetimes (more than 2 months) were obtained for these electrodes compared with the uranyl complex-based membranes. No differences in terms of response times and lifetimes were found for the 2 and 4% ionophore ratios. The selectivity behavior of the HL-based membranes (4% ionophore content) is shown in Figure 4. The order of selectivity presented by these electrodes was UO22+ . F- > Fe3+ > H2PO4- . Zn2+ > other ions. The main interferences were practically the same as in the UO2L2-based membranes. Even though only a significant interference from Fe3+ was expected, the effect on the electrode response observed by the fluoride and dihydrogen phosphate anions could only be attributed to the formation of some irreversible thin layer of uranyl complex at the membrane surface (these anions form complexes only with UO22+). The weak selectivity toward Zn(II) observed for these HL-based membranes could be related to the low values of the equilibrium constants (30) Helferrich, F. Ion-exchange; McGraw-Hill: New York, 1962. (31) Badr, I. H. A.; Meyerhoff, M. E.; Hassan, S. S. M. Anal. Chem. 1995, 67, 2613-8.

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Figure 4. Selectivity pattern of the HL-based electrodes with a 4% ionophore ratio (membrane D). ∆E corresponds to the change in potential from the starting potential (see text). The electrode was exposed to (1) UO22+, (2) Fe3+, (3) F-, (4) H2PO4-, (5) Al3+, (6) Cu2+, (7) Co2+, (8) Ba2+, (9) Ca2+, (10) Sr2+, (11) Ni2+, (12) SO42-, (13) Mn2+, and (14) Zn2+. Table 4. Selectivity Coefficients of the HL-Based Electrodes interference (Y) Mn2+ Fe3+ Zn2+ Co2+ Ni2+ F-

POT log kUO ( SD 2+ 2 ,Y

-0.76 ( 0.08 -0.78 ( 0.02 -0.9 ( 0.1 -1.2 ( 0.2 -1.32 ( 0.07 -1.44 ( 0.07

interference (Y) Cu2+ SO42H2PO4Ba2+ Al3+ Sr2+

POT log kUO ( SD 2+ 2 ,Y

-1.47 ( 0.05 -1.67 ( 0.01 -1.93 ( 0.05 -1.98 ( 0.07 -2.05 ( 0.04 -2.23 ( 0.04

presented by Zn(II) complexes.18 However, it is important to note that, in general, interferences showed less effect on the electrode response in the HL-based membranes compared to UO2L2-based electrodes. In this last case, interferences are expected to result from interactions with either the metal or the ligand or both. The POT potentiometric selectivity coefficients, kUO 2+ , for all the ions 2 ,Y studied are presented in Table 4. No response was obtained for other ions studied (Na+, Cl-, Cr3+, K+, Mg2+, NO3-). FLOW INJECTION STUDIES Flow-through tubular electrodes based on the bifunctional chelating agent (HL) and its uranyl complex (UO2L2) were also prepared for the determination of uranyl ion using flow injection techniques. The same plasticizer (DOS) and anion excluder (KTCPB) were also used in flow experiments. Moreover, from the data obtained for conventional configuration studies, the choice of ionophore ratios (either 2 or 4%) was irrelevant in flow-through tubular electrodes. However, a 4% ionophore content was finally used in order to avoid a decrease in terms of slopes or lifetimes.21,27 UO2L2-Based Tubular Electrodes. Membranes of type D (see Table 1) were used in all the experiments. Preliminary studies were carried out to determine the effect of the membrane support on the electrode response. Thus, membranes based in this ionophore showed linear sub-Nerstian responses (14-22 mV/ decade) for the total uranium concentration range 1.0 × 10-31.0 × 10-1 M (under the experimental conditions, this corresponds to a free uranyl activity range of 7.9 × 10-4-2.0 × 10-2 M) when

Table 5. Effect of the Carrier Solution Composition on the Calibration Parameters for the Flow-Through Tubular UO2L2- and HL-Based Electrodes HL-based electrodes

UO2L2-based electrodes, graphite composite carrier solution 1.0 × 10-3 M HNO3 1.0 × 10-3 M HCl 1.0 × 10-3 M HClO4 0.100 M (K+, H+)NO3- (pH 3) 0.100 M (Na+, H+) Cl- (pH 3) 0.100 M (Na+,H+)ClO4- (pH 3) a

graphite composite

silver epoxy resin

slope (mV/dec)

linear activity range (M)

slope (mV/dec)

linear activity range (M)

slope (mV/dec)

linear activity range (M)

14.0 10.1

7.9 × 10-4-2.0 × 10-2 1.0 × 10-5-7.9 × 10-4 a b 3.2 × 10-4-2.5 × 10-2 a

81.2 83.3 74.5 18.6 40.1 21.2

4.0 × 10-5-6.3 × 10-3 4.0 × 10-5-6.3 × 10-3 4.0 × 10-5-6.3 × 10-3 3.2 × 10-6-3.2 × 10-4 3.2 × 10-6-3.2 × 10-3 3.2 × 10-6-3.2 × 10-3

35.2 36.8 31.8 5.0 12.3 8.6

1.0 × 10-4-2.5 × 10-2 1.0 × 10-4-2.5 × 10-2 1.0 × 10-4-2.5 × 10-2 3.2 × 10-6-3.2 × 10-3 3.2 × 10-6-3.2 × 10-3 3.2 × 10-5-3.2 × 10-3

8.3

No linear response. b No response.

the graphite composite was used as the membrane support. In the case of the silver-loaded epoxy support, anomalous responses (partly erratic, partly anionic) were obtained, probably due to the interaction between the silver from the support and one of the components of the membrane. For this reason, heterogeneous graphite-based resin was used in further studies. The composition of the carrier solution also strongly affects the electrode response. Thus, results from the six different carrier solutions (see above) indicated that 1.0 × 10-3 M HNO3, 1.0 × 10-3 M HCl, and 0.100 M (Na+, H+) Cl-, pH 3 carriers presented sub-Nernstian responses with slightly better results in terms of slopes in the case of 1.0 × 10-3 M HNO3 (see Table 5). For this reason, the next experiments were performed in this medium. The lower slope values and the anomalous response observed in the 0.1 M carrier solutions could be attributed to KTCPB is electroactive with respect to alkali metal ions and to the influence of the background electrolyte anions in the aqueous phase.14 The effect of the injection volume was also studied. From the different volumes used (50, 100, 150, and 200 µL), the best results in terms of slopes and linear range were obtained for 100 and 150 µL. Similar slopes as before were obtained in both cases, although some differences in the operational range were observed. In the case of 100 µL, the linear range was 1.0 × 10-4-1.0 × 10-2 M (1.0 × 10-4-5.0 × 10-3 M in activity values), while for 150 µL, a 1.0 × 10-3-1.0 × 10-1 M (7.9 × 10-4-2.5 × 10-2 M) operational linear range was obtained. Potentials were always reproducible within (2 mV for three repeated injections at a given concentration level. The data obtained from the calibration recording indicated sampling throughput rate values in the range of 30-45 h-1, depending on the sample concentration level. The operational lifetime (around 20 days) was the same as obtained in batch studies. This would indicate that this parameter depends more on the stability of the membrane components than on the dynamic conditions used in flow experiments (usually shorter lifetimes are observed).27 HL-Based Tubular Electrodes. First studies performed with this ionophore showed poor linearity and narrow operational ranges. Several membranes at different KTCPB composition were prepared (membranes D-F; see Table 1). Results indicated that a wider operational range and higher slopes were observed for E membranes. This agrees with the literature where it has been reported that an increase in KTCPB concentration in the membrane above a certain limit provokes a narrowing of the linear range.14

The type of membrane support also strongly affects the electrode response, as was observed for the UO2L2-based tubular electrodes. However, for HL-based electrodes, no anomalous responses were obtained for either membrane support. Thus, membranes based on the graphite composite showed superNerstian responses (about 70-85 mV/decade) in the range 5.0 × 10-5-1.0 × 10-2 M (4.0 × 10-5-6.3 × 10-3 M in activity values). In the case of the silver-loaded epoxy support, Nerstian responses (29-35 mV/decade) were obtained, shifting the linear range to higher values: 1.0 × 10-4-5.0 × 10-2 M (1.0 × 10-4-2.5 × 10-2 M). Depending on the analysis requirements, advantages could be found in selecting graphite composites or silver epoxy resins for each particular application. The effect of the composition of the carrier solution was also studied in UO2L2-based electrodes. Similar results were obtained in the case of 1.0 × 10-3 M HNO3 and 1.0 × 10-3 M HCl for both electrode supports (see Table 5). However, 1.0 × 10-3 M HNO3 was considered for further experiments in order to compare with UO2L2-based tubular electrodes and with batch experiments. A decrease in terms of slope values was also observed for the 0.1 M carrier solutions, which is consistent with the same effect noticed for UO2L2-based tubular electrodes (see above). Injection volumes were also optimized, obtaining the best results for 100 and 150 µL as well. Better reproducibilitysthan UO2L2-based electrodesswas observed ((1 mV for three repeated injections at a given concentration level). Both types of electrodes, graphite and silver epoxy support, showed the same sampling rate (around 25 h-1). The slower responsescompared with UO2L2-based electrodessshows the same behavior as seen for the conventional configuration electrodes. This effect was also observed in the case of lifetimes, and electrodes showed longer operational lifetimes as well (more than 3 months). CONCLUSIONS This work has demonstrated that it is feasible to prepare uranium(VI)-selective electrodes using a bifunctional chelating agent (HL) or its uranyl complex (UO2L2) as ionophore. The final selection between the two sensor systems should be made depending on the accuracy, detection limits, interference background, etc., required by the analysis. However, it is necessary to point out that the response behavior of the electrodes developed in this work is comparable to the best analytical characteristics presented by other electrodes described in the literature.5-8,10-14,24 Analytical Chemistry, Vol. 72, No. 7, April 1, 2000

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The use of a bifunctional chelating agent as ionophore, incorporating two functional groups of a synergistic mixture in a single extractant, may replace the use of the two components in the sensing mixture. Results indicate a more effective interaction with the analyte, related to an improvement in terms of detection limits, in the case of having a unique molecule incorporating the two functional groups immobilized into a polymeric membrane, rather than the two separated synergistic ligands. In the later case, a lack of appropriate simultaneous interaction between the two ligands and the metal would exist. On the other hand, this work also demonstrates the importance of the fundamentals of solvent extraction behavior for the development of sensors and of structure-backed syntheses based on the experience from the sensor response. Moreover, the feasibility of preparing flowthrough tubular electrodes makes these uranyl-selective electrodes a convenient tool for continuous monitoring.

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Analytical Chemistry, Vol. 72, No. 7, April 1, 2000

ACKNOWLEDGMENT This work was supported by a grant from the Spanish Commission for Research and Development, CICYT (QUI99-0749CO3-02). Carlos Va´llez and Marina Prades are acknowledged for helping in part of the experimental work. The authors thank the Israel-Spain Exchange Program, sponsored by the Israel Ministry of Science and the Spanish Ministry of Education and Science, for their support.

Received for review July 21, 1999. Accepted December 29, 1999. AC990806L